1. Field of the Invention
The present invention relates to measurement of the spacing between and/or direction from a first object or location to a second object or location and particularly to determining the position of a vehicle with respect to a fixed position or another vehicle. More specifically, this invention is directed to apparatus for providing information commensurate with the distance between, relative velocity of and direction angle between adjacent vehicles or a vehicle and a fixed object. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is especially well suited to determining the relative positions of mobile vehicles and particularly the relative distance between two vehicles in motion. In the prior art a number of methods are used to determine vehicle position. The nature of the application, and in particular the measurement accuracy requirements and the environment, usually influence the choice of a position determination technique from the available alternatives. Thus, for a rail-type public transportation system the parameters of inter vehicle distance and relative velocity are particularly important for control system "following" modes and collision avoidance. Position angle measurement is obviously not as important as measurement of the distance between vehicles. For air traffic control or aircraft collison avoidance applications, on the other hand, both distance and direction information are of critical importance.
There are, in the prior art, three common methods for measuring range using electromagnetic energy transmission. A first method involves measurement of the round-trip propagation time of a signal which traverses the path to be measured in both directions. This first method makes use of radar or radar beacon techniques wherein distance is determined by measurement of the elapsed time for two-way transmission to appropriately located targets or transponders. Employing radar techniques, velocity may be determined by Doppler frequency measurement. A second prior art range measuring technique involves the use of precision clocks located at either end of the path whose length is to be measured; the clocks operating in conjunction with a signal transmitted in one direction over the path. In measurement techniques which utilize a clock synchronizing mechanism, an important characteristic of the transmitted signal is a unique relationship between a "time signature" and the time of transmission which allows a time difference measurement at the receiving end of the path whose length is to be measured. Such prior art methods for providing distance and velocity measurement by means of one-way transmission require expensive atomic clocks or complex synchronizing clock mechanisms.
A third prior art range measuring technique involves trilateration. Trilateration measurement schemes, for example the well known Gee and Loran long range navigation systems, are limited in their range measurement resolution and utilize comparatively expensive and complex radar-time measurement techniques and apparatus.